Paper is hydrophilic material and by nature, tends to have an affinity to water because many hydroxyl groups of cellulose exist therein, so that it absorbs water to swell. For this, a hydrophobic material (mainly, a neutral sizing agent) preventing bleeding is added during the course of producing paper for the purpose of providing water resistance to a printing paper. Also, a white inorganic pigment (for example, clay and calcium carbonate) is added to printing papers to provide whiteness and transparency. Because the grain size of the pigment is smaller than the irregularities of fibers, surface roughness formed by fibers remains on the surface of the paper and therefore, printing accuracy is limited if the paper is used as it is for printing. The surface of the paper is coated with a pigment having a grain size of about 0.1 to 0.3 μm and primarily containing kaolin and calcium carbonate to smooth it. However, a binder such as a synthetic latex used to fix this pigment fills clearances between the pigment grains to hydrophobicize the surface. The surface of the coated paper is smoothed to be hydrophobic in this manner. Heavy calcium carbonate (one obtained by milling and classifying natural lime stone) is usually used for coated paper such as art paper, coat paper, gravure paper and internally sized paper such as high-quality paper, middle-quality paper, and rice paper. Many uncoated papers and coated paperbase contain calcium salts such as calcium carbonate and almost all coat layers of coated papers also contain calcium salts such as calcium carbonate.
Ink jet printing systems have been developed and widely put into practical use in paper printing fields.
When an inkjet printer is used to print, ink droplets discharged from a print head are landed on a printing paper, penetrated and fixed to form dots, with a lot of these gathering dots, an image is formed. This process of formation of dots is important to form a clear image.
When a concept “bleed rate” which is a value showing how many times the diameter of a dot becomes as that of a discharged ink droplets is used, the bleed rate b is given by b=D/d when the diameter of the discharged ink droplet is d and the diameter of a dot on the printing paper is D.
Because it may be considered that the formation of dots on printing paper is divided into a landing phenomenon and a penetration phenomenon, the following equation can be described: b=(D/D′)×(D′/d), using the diameter D′ of a dot when a droplet is landed on the printing paper.
Here, when landed spread rate s and penetration spread rate p are defined as s=D′/d and p=D/D′, the bleed rate b may be given by b=s×p (see, The imaging Society of Japan, “Inkjet”, Tokyo Denki University Press, September 10 in 2008). Therefore, in order to form a clear image, it is necessary to improve fixability by modifying a pigment, resin emulsion and the like in an ink composition to thereby prevent the penetration spread rate p from being increased.
Also, when an ink composition is penetrated into plain paper, such a phenomenon (feathering) occurs that the periphery of an image becomes fluffy, reflecting the mesh of cellulose fibers forming the plain paper. Because this phenomenon more or less occurs when ink penetrates into paper, it is necessary to limit the penetration by conditioning the composition of an ink to avoid feathering. If the penetration is limited on the other hand, this requires a certain time for the ink to be fixed, which is sometimes a hindrance to high-speed recording. In the design of ink, in order to attain the compatibility between feathering and fixing time, the ink needs to be fixed by adjusting the ink penetration.
Various papers such as coated paper and uncoated paper are used as the paper medium to be used as a printing paper. However, in the case of uncoated paper, a coloring material is not retained on the surface of the paper because ink is easily penetrated into paper fibers, posing the problem that paper bleeding is easily caused, so that only insufficient density is obtained. In the case of coated paper, on the other hand, ink is scarcely penetrated and a coloring material is scarcely fixed because a coat layer exists on the surface of the paper, the ink tends to bleed so that a clear image cannot be obtained.
In light of this, a denatured pigment obtained by binding a functional group, having a predetermined calcium index, to the surface of a pigment is proposed, to improve the fixability of the coloring material to paper (see, for example, Patent Literatures 1 and 2).
Here, the calcium index indicates a measure of ability to coordination-binding dissolved calcium ions, that is, ability of the functional group which captures the calcium ions. The larger the calcium index is, the more strongly and more effectively the functional group coordination-binds the calcium ions.
Also, many uncoated papers and coated papers contain calcium salts such as calcium carbonate and many coat layers of coated papers contain calcium salts such as calcium carbonate.
The above patent literatures describe such an inference that when an ink composition containing a denatured pigment as mentioned above is printed on paper, the denatured pigment interacts or binds with a calcium salt or other divalent metal salts existing in or on the surface of the paper, with the result that a coloring material is easily fixed to the paper.